Biting and nipping are unacceptable in any form. They are the one "Cardinal Sin", if you will, that I have. Jumping up would be a close second.
Generally, but certainly not always, it begins as a puppy. Who can resist wrassling around with a Vizlsa puppy and engaging in fun time with them, but we tend to downplay the little nips at that age as just being puppyish behavior that they will outgrow. In reality though we are establishing the boundries of acceptable behavior at the most critical time in their life. Simply put, it has been impressed upon them that biting is an acceptable behavior. Now this doesn't mean it's a guarantee that if they are never allowed to nip and bite as puppies, they'll be perfect, far from it. If however it is impressed upon them at an early age that nipping and biting are unacceptable, when they get older the behavior can be more easily addressed, enforced, and corrected as there has been a foundation already established, and now we're just waiting for maturity to kick in. In my mind, and my experience, if they are never allowed to start nipping and biting, then the behavior is mitigated at an early age.
I am going to fly in the face of current convention in dealing with older dogs, no longer puppies, that still nip and bite.
No matter what term we use to identify the behavior, such as resource guarding, or dominance behavior attributes, it is just bad manners and should not be tolerated.
When Gunnr nips me, or even puts her teeth on me during play. I immediately grab her muzzle, give it a sharp shake, command "No Teeth" while slightly pinching her gum, and then put her in either her kennel, or restrain her for a minute, while I hold her muzzle and talk to her. There is no slack here, ever. She knows exactly where those teeth are.
I know this sounds harsh, but it is all very controlled, and I don't believe in using pain as a tool. Discomfort yes. Pain, no.
Since I started with Gunnr, last August, she has really improved. She does still do it occasionally, but all I usually have to say is no teeth, and we can get on with it. In the beginning she was out of control, and was going to unintentionally hurt someone. She bit hard. Hard enough that the nerves in my hands would go numb when she caught my hand or arm
Age has helped a tremendous amount also. She genuinely want to please now and tries not to be bad, but sometimes she is still Gunnr, and can't help herself.
I predict that by the age of two, the behavior will be gone. She's really matured a lot these last few months.
It takes time and commitment. It has to completely black and white with dogs. They can never get away with putting their teeth on you, no matter what. Every incidence has to be met with correction. Don't hurt them, don't beat them, but very firmly and under control correct the behavior in a patterned, consistent manner so that your dogs always see the same picture. Also try to ensure that whomever else handles our dog reinforces the behavior in approximately the same manner. The clearer the picture, the faster they'll get it.